It is a wasting because it could be done with staking, that way the could shouldn't need miners.
Yes, but it could end up making Bitcoin centralized. Think about it. Imagine being a huge exchange with a lot of PoS coins in custody? Solely from being a whale, you automatically have a significant influence over the network. In contrast to PoW whereas you'd actually have to dedicate money to buy hardware.
No need to imagine, it's already been done -- at least with Delegated POS.
https://twitter.com/VitalikButerin/status/1234522463129800709Poloniex, Binance and Huobi were caught staking customer STEEM and using it to attack the existing DPOS consensus. They've done a pretty good job of brushing the attack under the rug, but the evidence is rather damning.
POS hasn't been solved yet. It may never be. POW is the best we have, for now and the foreseeable future.